What is a White Hole?

A friend recently mentioned something called a White Hole as opposed to a Black Hole. I see you had a few posts on Black Holes but couldn't see anything on White Holes. So my questions would be: What is a White Hole? Do they exist? - Question posed by Jen.

What is a White Hole?

A White Hole, in principle, is the exact opposite of a Black Hole: a Black Hole swallows up energy and matter from the universe over time; a White Hole spews out energy and matter into the universe. Imagine recording a video of a Black Hole doing its stuff, then run the tape backwards. What you're watching now is a White Hole.

That was the easy bit...

Do they exist?

I could probably get away with just saying "no" here, and leaving it at that. But that wouldn't be any fun, would it, and besides it wouldn't be completely true. Closer to the truth would be if I said "probably not." That's pretty close to "no", but the word 'probably' opens us up for more discussion...

The maths behind Black Holes and Relativity says that White Holes can exist. In fact, if you set up the most basic mathematical description of a Black Hole, that is one without any mass, and crunch the numbers, a White Hole just pops up as part of the answer: it has to exist. "But!" I hear you cry, "a Black Hole has to have mass! That's how they're formed- great wodges of stuff collapsing in on themselves! You told us that right here!"

Technically that's true. Technically. To have a Black Hole, you need a singularity. The best way to make one of those is to put huge amounts of matter in the same place until it all collapses in on itself. In fact, the only way we know of to have a singularity without any mass in it is if they came ready-made at the beginning of the universe.

So if the universe just happened to start out with singularities in place (we have no reason to think that this may be the case), then there could be massless Black Holes, with corresponding White Holes.

The problem now is that if any matter enters the Black Hole, even the smallest speck of dust, it is no longer massless, and the corresponding White Hole poofs out of existence. The universe has been around for quite a while now, so the chances of any massless Black/White Holes still hanging around (assuming that there were any in the first place) is very, very tiny.

My brain hurts now. Anyone for some light relief? This is a related scene from a favourite sci-fi sitcom of mine from the early 90s called Red Dwarf. The episode it's from is called White Hole. The science only really gets it right in places by accident, but it's entertaining: